1. Field of the Invention
The present invention relates to a method of measuring a length of sections of extrados or intrados curves of an elongated workpiece. Further, the invention refers to a relevant instrument, which is in the following an encoder that is mounted on a pyramidal bending machine only by way of example. Of course it should be understood that the method can be embodied also on a pipe bender different from the pyramidal roller bending machine.
2. Description of Related Art
When an elongated workpiece has to be bent according to varying curvature radii in order to obtain for example fixed radius sections spaced by different radius bends or in general varying radius bend sections and such an elongated workpiece has to copy exactly an already existing contour or profile, on which the elongated workpiece has to be adjusted, it is important that the bend sections have measures accurate as much as possible in length.
A contact length measuring device, generally an encoder, is currently used that is positioned along a pipe to be bent in a symmetrical point in the bending machine, i.e. on the projection of the axis of a deforming roller. However, this symmetrical cross-section of the pipe is not deformed upon the bending stress.
As known, an elongated workpiece, such as a pipe or other section iron to be bent, is a Saint-Vènant solid in cross-sections different from this symmetrical or neutral cross-section. Then, the Saint-Vènant solid's cross-section is divided by a separation line called neutral axis in two parts, one of which is extended and the other is compressed by the bending stress as applied from outside during a bending operation. Therefore, it happens that the extrados has a length major than the intrados due to the stretching caused by the bending. This stretching increases when increases the height of the elongated workpiece and decreases the curvature radius, and further it depends on the speed with which the bending of the elongated workpiece is performed.
Finally, even if most sophisticated means are used for controlling the bending operation such as laser means, it is important to check moment by moment the behaviour of the pipe in the bend sections being performed and in the projection thereof in a plane. In such a way it will occur less frequently that the manufactured workpieces do not match the expectations.